An electron gun assembly for color picture tube has a function of generating an electron beam and focusing and accelerating the generated electron beam according to an object. In particular, a focusing lens system formed by a plurality of electrodes becomes an important element dominating the performance of the color picture tube.
The focusing lens system of the electron gun assembly for color picture tube functions to simultaneously focus three electron beams respectively corresponding to red (R), green (G), and blue (B). A bi-potential focus lens and a uni-potential focus lens are examples of a fundamental lens form of such a focusing lens system. As a matter of fact, a combination of these fundamental lens forms is utilized in order to improve the focusing performance. For example, various composite lens systems such as tri- potential focus type (abbreviated to TPF type), multi-step focus type (abbreviated to MSF type), and quadra-potential focus (abbreviated to QPF type) are utilized.
FIG. 1 is a diagram showing the schematic structure of a QPF type electron gun assembly described in Jpn. Pat. Appln. KOKAI Publication No. 54-72667.
The electron gun assembly includes a cathode 10, a first grid 11, a second grid 12, a third grid 13, a fourth grid 14, a fifth grid 15, and a sixth grid 16 disposed in the cited order along the same axis. Each grid has an electron beam passing hole which passes an electron beam emitted from the cathode 10.
The cathode 10 and the grids 11 through 16 are applied with respective predetermined potentials. The cathode 10, the first grid 11, and the second grid 12 emit thermions and form crossovers of electron beams. The second grid 12 and the third grid 13 form a pre-focus lens 17 to focus electron beams crossed over preliminarily. The third grid 13, the fourth grid 14 and the fifth grid 15 form an auxiliary lens 18. The fifth grid 15 and the sixth grid 16 form a main lens 19.
Recently, color picture tubes are required to be larger in size and higher in definition. The electron gun assembly is also required to have shorter inter-electrode distance values and higher precision. In particular, a triode ranging from the cathode 10 to the second grid 12 was formed so as to have relatively small inter-electrode distance values, but recently the inter-electrode distance values tend to become still smaller. As the inter-electrode distance becomes shorter, not only the assembling error of each inter-electrode distance but also inter-electrode distance changes caused by the influence of heat of a heater provided for the cathode 10 need to be made smaller.
As the second grid 12, a plate thicker than that of the first grid 11 is typically used. Thus, the heat capacity of the second grid 12 becomes large. After the heater of the cathode is ignited, it takes time until thermal stability is attained. Thus, the white balance immediately after the ignition of the heater tends to break down.
In order to solve this problem, there is disclosed in Jpn. UM Appln. KOKAI Publication No. 57-128755 an electron gun assembly including a second grid 12 having a thick flat plate 21 with a predetermined opening formed therethrough and a support 22 for fixing the thick flat plate 21 to bead glass 20 as shown in FIG. 2. The support 22 of the second grid 12 is curved toward a side opposite to the support side of the thick flat plate 21. In the structure of the second grid 12, the thick flat plate 21 is not directly fixed to the bead glass 20 and consequently the area of the thick flat plate 21 can be made small. As a result, its heat capacity can be made small and consequently it becomes possible to prevent the inter-electrode distance from being changed by thermal expansion.
However, the support 22 of the second grid 12 is disposed on the side of a third grid 13. For providing the distance between the second grid 12 and the third grid 13 with a predetermined value, therefore, it is necessary to make a portion of the third grid 13 located on the side of the second grid 12 smaller than an inside diameter 23 of an opening portion of the second grid 12 located in the support portion 22 and adopt such a structure that a face 24 of the third grid 13 opposed to the second grid 12 is surrounded by the support 22 of the second grid 12.
Conventionally, a portion of the third grid 13 located on the side of the second grid 12, i.e., an electrode of a third grid bottom is formed so as to have a cup-shaped structure as shown in FIGS. 3A through 3C or a cup-shaped structure as shown in FIGS. 4A through 4C.
FIG. 3A is a top view of an electrode seen from the side of a cathode 10. FIG. 3B is a sectional view of the electrode seen from an in-line direction, i.e., the horizontal direction. FIG. 3C is a side view of the electrode seen from a direction perpendicular to the in-line direction, i.e., the vertical direction. A bottom face 30 of the cup-shaped electrode shown in FIGS. 3A through 3C takes the shape of an approximately rectangle having longer sides in the horizontal direction. Furthermore, so as to make the shape of an opening portion 31 substantially the same as that of a bottom face 30, the longer sides of the bottom face 30 are joined to longer sides of the opening portion 31 with side walls 32 extended in the tube axis direction.
FIG. 4A is a top view of an electrode seen from the side of the cathode 10. FIG. 4B is a sectional view of the electrode seen from the horizontal direction. FIG. 4C is a side view of the electrode seen from the vertical direction. The electrode shown in FIGS. 4A through 4C has projections 33 respectively for individual electron beam passing holes.
FIG. 5 is a sectional view of a part of an electron gun assembly having the cup-shaped electrode shown in FIGS. 3A through 3C on the bottom of a third grid seen from the horizontal direction. In this shape, the distance between a folded portion 34 of a support 22 of the second grid 12 and a side wall 32 of the bottom of the third grid 13 is small and the withstand voltage characteristics is poor. In other words, the distance between the folded portion 34 and the side wall 32 is small, and in addition a large potential difference is formed between them. This results in a problem that a leak tends to occur.
Therefore, it is conceivable to use an electrode having a narrowed width of the bottom face in the vertical direction as shown in FIG. 6A through 6C. FIG. 6A is a top view of the electrode seen from the side of the cathode 10. FIG. 6B is a sectional view of the electrode seen from the horizontal direction. FIG. 6C is a side view of the electrode seen from the vertical direction. If the electrode shown in FIGS. 6A through 6C is used, the distance between the folded portion 34 of the second grid 12 and the side wall 32 of the third grid 13 can be widened, and consequently the problem of the leak is eliminated. Since the inside diameter of a side of the opening 39 of the third grid bottom becomes small, however, an electric field 36 of the auxiliary lens penetrating from the side of the fourth grid 14 to the side of the third grid 13 is affected. Thus there occurs a problem that a lens which is asymmetric in the horizontal direction and the vertical direction is formed. As a result, a beam spot formed on a screen does not take the shape of a circle but takes a distorted shape.
If the electrode taking the shape shown in FIGS. 3A through 3C or FIGS. 6A through 6C is used, either the withstand voltage characteristics or the auxiliary lens characteristics are sacrificed.
Furthermore, if the third grid bottom takes the shape shown in FIGS. 4A through 4C, then the distance between a support 22 of the second grid 12 and a third grid side wall portion 37 is widened, and consequently the withstand voltage characteristics are improved. Furthermore, since an opening side 38 of the third grid bottom can also be widened, the influence exerted upon the auxiliary lens can be decreased. Since the projections 33 are disposed respectively for the individual electron beam passing holes, the shape becomes complicated. Furthermore, individual position precision between the projections 33 and the electron beam passing holes becomes necessary not only in the vertical direction but also in the horizontal direction. As a result, the manufacturing becomes difficult, and there is a fear of an increase in cost.
In the conventional electron gun assembly, and in particular in the electron gun assembly of QPF type, the thick flat plate of the second grid is fixed to the bead glass by using the support which takes such a shape that the support is folded to the third grid side as described above. The method poses a problem that the withstand voltage characteristics are degraded or the electric filed characteristics of the auxiliary lens formed between the second grid and the third grid are affected, depending upon the shape of the part of the third grid located on the second grid side. Furthermore, if it is attempted to solve these problems, the shape of the electrode becomes complicated and there is a fear of an increased cost.